When produce such as potatoes, onions and other crops are harvested, the produce varies greatly in size, shape and weight. Different markets and customers often have disparate needs for produce of a given dimension or attribute. Frequently, produce of a given size or shape is in higher demand and will command a higher price. Consequently, harvested produce must be segregated according to size, shape or some other attribute to meet the needs of the customer and the market. Once sorted or graded, the produce of a specific size or characteristic may be shipped to the customer or market demanding that size or characteristic.
Potatoes, onions and other subterranean produce often have large amounts of soil and small rocks adhered to the produce when harvested. This material must be removed before the produce is weighed for sale. Soil removal can become difficult when the soil is relatively moist because the mud will adhere to equipment sometimes causing it to clog and sometimes causing it to become lubricated with a film of mud.
Potato harvesters often use a grading apparatus which comprises a succession of closely spaced roller shafts containing rollers with protrusions thereon which act to propel the potatoes along a grading path. The first roller shafts encountered by the potatoes are typically closely spaced with radial protrusions which serve to turn and propel the potatoes thereby loosening soil, rocks and mud which may have adhered to the potatoes. Subsequent sets of roller shafts are spaced increasingly further apart to allow the smaller and then larger grades of potatoes to pass between the rollers and fall into grading containers or conveyors for each grade of potato.
Problems may arise with this type of grader or sorter when excess mud is allowed to build up on the surfaces of the rollers. One problem occurs when mud builds up on the roller surfaces which define the opening through which the potatoes are graded. Mud buildup causes an effective decrease in the opening size and therefore a decrease in the potato size passing through the grader opening. Another problem occurs when roller surfaces become lubricated and clogged with mud such that potatoes are not properly propelled along the grader path. This causes potatoes to build up on the grader without forward movement. If this condition persists, potatoes accumulate and spill over the sides of the grader path putting a halt to grading operations.
Another problem occurs when grading rollers do not have proper elastic deflection during operation. Rollers and protrusions therefrom must deflect in a direction parallel with the grading path so as to prevent excessive impact forces on the produce. However, the protrusion must be sufficiently rigid to propel the produce along the grading path. Roller protrusions must also have sufficient lateral rigidity to retain a uniform shape through grading openings as produce is propelled across the openings. Proper protrusion rigidity contributes to uniform grading and efficient produce propulsion.
In reference to FIG. 1, a prior art roller may be used to illustrate the problems of the prior art. Prior art rollers, such as the "acorn" roller of FIG. 1, have tightly spaced ribs 2 and nubs 4 which create narrow crevices 6 where mud and dirt can easily accumulate. The concentration and density of the ribs 2 and nubs 4 also produces non-aggressive propulsion characteristics with no defined protrusions to grasp and propel the produce along the grading path. This lack of propulsion results in diminished production rates, especially in muddy conditions when the rollers are lubricated with mud.
The easy accumulation of mud and dirt in the many crevices 6 on the "acorn" roller also results in inaccurate sizing. In reference to FIG. 2 where a prior art "acorn" roller shaft layout is shown, it can be seen that these prior art roller are configured in such that the centerline 10 of each roller on a first shaft 14 aligns with the centerline 12 of each roller on the next successive shaft 16 thereby creating a somewhat distorted hexagonal pocket 20 or opening between rollers 14 and shafts 16 and 18.
Produce of a particular size or grade will fall within this opening 20 unless dirt and mud build up on the surface of rollers 14 thereby decreasing the size of opening 20 and consequently the size of the produce falling through the opening 20. It can also be seen that produce slightly larger than opening 20 will rest over opening 20 without being propelled down the grading path if an aggressive protrusion pattern is not available. This is often the case when tightly spaced ribs 2 and nubs 4 clog with dirt and mud.